In order to detect the level of the liquid contained in closed devices such as tanks, boilers and the like, use is advantageously made of magnetic level indicators having a float which moves axially in a tubular body communicating at its ends with the receptacle whose level is to be indicated, which float bears a permanent magnet designed to orient a plurality of magnetic indicator dipoles, whose position show the level in the receptacle.
A level indicator of this type is described in U.S. Patent No. 4,483,193.
This level indicator is particularly reliable and secure in most cases, since it contains no transparent components which may be of low strength and which often make it difficult to see the existing level, since the unit is sealed from the outside in all the cases. In some cases, however, which are particularly problematic due to the high temperature and pressure of the liquid contained in the recipient whose level is to be indicated , this type of indicator may also be brought into a critical condition.
The float bearing the magnet, which must be light to float, must be able to operate in these conditions as well without being degraded by the external pressure and the high temperature.
An example of such situations in provided by the cylindrical bodies of steam generator boilers, for instance in thermoelectric plants, which operate with water at a temperature which may reach 370.degree., corresponding to an equilibrium pressure of 220 bar, together with a water density of some 0.5 g/cm.sup.3. Under these conditions, the structural strength of a float which has to be very light in view of the density of the water, is not enough to prevent its degradation as a result of instability due to external pressure. It is possible to use pressurized floats, i.e. floats which have an internal pressure which partially balances out the external pressure in working conditions, but even these floats are not suitable for extreme conditions, since they must be pressurized to a level which is too high at ambient temperature if they are to oppose the external pressure at the working temperature efficiently.
It is also possible to use, within the float, a quantity of an evaporating liquid, whose vapor pressure increases rapidly with temperature, for example the same as the external liquid, but this internal liquid must be used in extremely small quantities so that it is not detrimental to the ability to float. These systems have the further problem that during the heat transients, when the temperature within the float is different from that outside it, there is no longer an equilibrium between the internal and external pressures which may lead, in these conditions, to the degradation of the float structure.